The present invention relates to the preparation of milk froth or foam for cappuccino (milk-coffee beverage) by means of superheated steam.
Espresso machines are customarily equipped with devices for generating superheated steam and with a steam discharge pipe for producing milk froth.
To produce milk froth, the outlet opening of the steam pipe which has a reduced cross section and is thus configured as a nozzle, is manually positioned in a vessel filled with milk so that it just contacts the milk surface or slightly dips into the milk. Once the steam cock is opened, the steam jet exiting from the nozzle carries air along from the environment and a steam-air-milk mixture is produced at the liquid interface resulting in the desired milk froth.
This process requires practice and sensitivity on the part of the user since, in order to continuously produce milk froth, the nozzle must always be held in an optimum position relative to the milk surface, which is possible only by skillfully moving the milk containing vessel up and down.
Particularly for home espresso machines, devices for simplifying the preparation of milk froth have therefore been developed and disclosed in which by means of an accessory device for the steam outlet nozzle, the three-component system of steam, air and milk can be produced in such a way that the permanent search for the operating point is no longer required.
One of the prior art devices (DE 3,538,041.A1, Int. Class A 47J31/44, corresponding to U.S. Pat. No. 4,735,133) is composed of a cylinder that is closed at the top and open at the bottom and is placed around the outlet nozzle in the manner of a casing and in which the steam exiting from the nozzle creates subatmospheric pressure. If the bottom portion of the cylinder, which has bores in its casing surface, is immersed below the liquid level of a vessel filled with milk, milk enters the cylinder through the bores and floods the nozzle. Due to the subatmospheric pressure, air is sucked in through a further bore in the upper portion of the cylinder, which remains above the liquid level, and the three-component system of steam, air and milk, and thus the milk froth, is created in front of the steam nozzle within the cylinder to then exit from the cylinder opening. The drawback of this device is that the resulting milk froth has relatively large pores and its consistency is influenced by the fat content of the milk.
In another prior art device (CH 0,673,212.A5, Int. Class A47 J, corresponding to U.S. Pat. No. 4,800,805) the lower open end of an air guiding device is positioned downstream of the exit nozzle in such a way that the outflowing steam surrounds it. The subatmospheric pressure thus created in front of the opening sucks air in through the air guiding device.
When this device is immersed below the liquid level of a vessel filled with milk, the air sucked in through the air guiding device, whose upper opening remains above the liquid level, also produces the steam-air-milk mixture and the milk froth. This device also operates in a very unstable manner, reacts with sensitivity to the sucked-in quantity of air and usually furnishes only large-pored milk froth. In a modified version of this device, the steam outlet pipe is provided with a hollow, downwardly open extension following its nozzle outlet into which open the steam outlet opening of the nozzle as well as the lower opening of the air guiding device which lies downstream thereof toward the open end of the extension. The outflowing steam produces a subatmospheric pressure in the hollow extension and air is sucked in through the air guiding device to be mixed with the steam in the extension. When this device is immersed below the liquid level in a vessel filled with milk, with the upper opening of the air guiding device remaining above the liquid level, the steam-air mixture generated in the extension, once it leaves the opening of the extension, encounters the milk, the steam-air-milk mixture is produced and thus the milk froth.
In a third prior art device (European Patent Application O,195,750.A2, corresponding to U.S. Pat. No. 4,715,274) which, in contrast to the above-described accessory devices, is not immersed into a vessel filled with milk but sucks milk from an external vessel through a conduit, the steam nozzle is followed by a cascade composed of a cylindrical condensation channel into which opens a milk intake conduit, and a mixing channel into which opens an air intake conduit. The mixing channel is followed by an emulsification chamber which has an outlet opening. In this arrangement, the steam flowing out of the nozzle produces a subatmospheric pressure in the channels connected to its outlet and step-by-step in succession the milk sucked in by the subatmospheric pressure is first mixed with the steam in the condensation channel, then, in the mixing channel, the steam-milk mixture is mixed with the air sucked in by the subatmospheric pressure through the air conduit and finally the mixture of the three components, steam, milk and air, is conducted into the emulsification chamber in which the milk froth is produced in a turbulence zone and subsequently exits through the chamber opening.
Although the above-described devices noticeably facilitate the preparation of milk froth, since their manipulation does not require any particular skill on the part of the user, this advantage is connected with a relatively high amount of technology expenditures since some expensive or precision manufactured components are required. Since in the chambers or channels connected to the outlet of the steam nozzle sudden changes in cross section produce Carnot surge losses once the steam leaves the nozzle and dead water zones downstream of the disturbance locations produce further flow losses within the chamber system, these devices require relatively high steam pressures to compensate for these flow losses and do not furnish a usable froth in conjunction with steam generators which generate only a low steam pressure. Even with high steam pressures their foaming result and the foaming time are dependent within narrow limits from the sucked-in quantity of air and the fat content of the milk.
Once the above-described foaming aids became known and were put to use, novel home coffee and espresso makers were developed in which, because of the systems involved, only small quantities of steam and low steam pressures are available. For the above-described reasons, the prior art foaming aids do not furnish a satisfactory operating result in connection with these devices.